The oozing sap symptom is a general one, called "gummosis". Trees produce the gummy sap in response to diseases or insect invasion. The curling leaves indicate aphid feeding. It's hard to spray the tops of trees unless they are very small but if you thoroughly spray the leaves with insecticial soap, you should be able to control the aphids. If you can't reach the top of the tree, just sit back and wait for ladybugs to find the aphid population and control it for you.